Maybe you should look at the question a bit differently. The elements your question is concerned with are all in the mid-field of their respective sentences (the mid-field is the part enclosed in square brackets):1
- Meistens kommen [pro Folge nur zwei Schauspieler] vor. (The verb is vorkommen.)
- Oft spielen [hier sehr bekannte deutsche Schauspieler] mit. (The verb is mitspielen.)
So, in essence, you are asking why the subject is at the end of the mid-field rather than at the beginning. That is a fair question. Since your second example is a bit ambiguous (see below), I'll focus on the first.
There is no fixed position for the subject within the mid-field. However, under the principles governing the unmarked order of the mid-field, the subject (if present) usually comes quite early. That is largely because the subject comes before other nominal complements and because the very first position within the mid-field (which might be empty, though) is reserved for non-nominal complements (including a pronominal subject) or a nominal subject. In practice, this is also frequently where you find it. But there is no rule saying that it must be the first element of the mid-field.
Your example contains one slight difficulty, which is the focus particle nur. Focus particles have long been known for their in some way rather restricted syntagmatic behaviour. The basic pattern - not completely without exceptions - is that unstressed focus particles in the mid-field immediately precede their domain if that domain is realised in the mid-field as well (see Sudhoff 2010:60). The domain here is the nominal phrase zwei Schauspieler, hence we should expect to find nur zwei Schauspieler. (Unlike in English: only two actors / two actors only.)
There is quite some liberty in positioning adverbials like pro Folge. A guiding principle is that items that are closer functionally or cognitively tend to be placed closer in the linguistic code (adjacency principle). Accordingly, if their scope is broad, supplements tend to be farther to the left of the mid-field; if their scope is narrower, they tend to be farther to the right. Here, placing pro Folge ahead of nur zwei Schauspieler underscores that it refers to/modifies the whole sentence; a position to the right would tend to narrow the scope down to the verbal expression. (This is not particularly important in the case of pro Folge. I would have difficulty even identifying a preferred order here.)
Your second example is ambiguous as hier may or may not be part of the nominal phrase. This has implications for any positional analysis since you cannot "rip apart" nominal phrases.
1 You write that in your examples there are certain elements "between the verb and the subject". That is not entirely correct as the vor and mit are part of the verb; the verbs are used in Distanzstellung ("distance position"), with the finite part before and the infinite part after the mid-field.